1. Field of the Invention
The present invention relates to a power-on circuit of a peripheral component, particularly to a circuit for reducing instant impulses when the power supply of a peripheral component is turned on.
2. Description of Related Art
Peripheral components in the modern industries, such as USB (Universal Serial Bus) devices, capture power supply from a USB host. When a USB host is connecting to a USB device, the USB host will provide 100 mA power supply current to the USB device. After the connection is established, the USB device demands the USB host to supply more current. But before the connection is established, due to the existence of filtering and stray capacitors, impulses caused by a power-on current always happen so that a Hub responsible for monitoring the power supply and building a connection protocol will mistake an overloading occurrence of the power supply and disconnect the USB device.
To resolve the above problem, a prior USB system joins a power-on apparatus between the power supply end and the USB device, shown as FIG. 1. The prior power-on apparatus mainly uses a resistor 16 and loading capacitor 15 to form a RC charging/discharging circuit for preventing current impulses, and uses a switch 14 to bypass the resistor 16 after a period of time to avoid a voltage drop caused by the resistor 16 in a normal connection. Besides, the prior power-on apparatus further comprises a switch 13 for conducting the USB power supply when the power-on signal turns on, and a delay circuit 11 turns on the switch 14 after a period of time.
FIG. 2 shows a circuit diagram of another prior power-on apparatus. Similar to the theorem with the prior circuit shown in FIG. 1, the prior power-on apparatus only uses a diode 21, a second capacitor 23 and a PNP transistor 22 to replace the switches 13 and 14 and a delay circuit 11 in FIG. 1. In FIG. 2, when the power-on signal turns on and output from a diode 21, a RC channel formed by the second capacitor 23 and resistor 16 will slowly enable the PNP transistor 22.
From the viewpoint of designing an IC, the area of a resistor is so large and wastes a chip area. Therefore, how to efficiently design a circuit that properly builds a connection protocol of the peripheral component and saves a chip area is an important issue.
The main object of the present invention is to provide a power-on circuit, which can avoid improperly shutting down a connection due to an impulse phenomenon when the power supply is being turned on.
The second object of the present invention is to provide a power-on circuit, which occupies little chip area.
The third object of the present invention is to provide a power-on circuit, which is easy to control the enabling time of the power supply.
To obtain the above purpose, the power-on circuit of the present invention comprises a switch control circuit for controlling the switching of a P-type or N-type transistor. For the P-type transistor, the switch control circuit includes a pull-high element, a current source and a current switch. For the N-type transistor, the switch control circuit includes a pull-down element, a current source and a current switch. The enabling time of the P-type or N-type transistors is controlled by the switch control circuit and the capacitor shunt with the switch control circuit so as to slowly enable the transistor switch. In other words, the present invention utilizes the slowly increasing or decreasing characteristic caused by charging the capacitor with the current source to control the enabling time of the P-type or N-type transistor switch so as to obtain the purpose of slowly enabling the power supply of the peripheral component. For designing an IC, it is easy to design a constant current source that occupies only little chip area, and the disadvantage of the prior art is thereby resolved.